1. Field of the Invention
The present invention relates to a metallic circuit board used as an electronic circuit board and a method of manufacturing the same and more particularly to a thin film diode array (e.g., active matrix type) applied to a liquid crystal display apparatus.
2. Description of the Related Art
Liquid crystal displays which are roughly classified into a single-matrix- and an active-matrix-type, recently attract a great deal of attention. In particular, since the active-matrix-type liquid crystal display apparatus is capable of perfect and independent switching of liquid crystal and of displaying a high quality image on a large image plane, the research and development thereof has been actively proceeded. Further, the active-matrix-type is mainly classified into two types. Namely, one is a liquid crystal display apparatus using an element called thin film transistor (hereinafter abbreviated as "TFT") and the other is that using an element called thin film diode (hereinafter abbreviated as "TFD"), and these two kinds of elements are called active element.
The liquid crystal display apparatus using a TFD is expected to steadily grow in the industry. This is because, though it is inferior to one using a TFT in image quality, it is inexpensive and suit for mass production owing to its simplification in construction.
An example of a conventional TFD array is shown in FIGS. 16, 17. FIG. 16 is a partial plan view showing a constitution of the TFD array. FIG. 17 is a sectional view taken on line V--V of FIG. 16.
The TFD array is manufactured as follows.
First, tantalum is applied by spattering to one surface of a glass substrate 21 to form a tantalum layer having a predetermined shape and the tantalum layer is anodized up to a predetermined depth from the surface of the layer to form an insulating film 24. Not-anodized parts of the tantalum layer form a lower electrode 22 and a lower wiring portion 23. The insulating film 24 can be also formed by the application of Ta.sub.2 O.sub.5 or SiN.sub.x by the use of a spattering or P-CVD method to the entire surface of the substrate coated with tantalum.
Further, an electroconductive film of a metal such as Al, Ti, ITO etc. or an oxide is laminated on the insulating film 24 on the lower electrode 22 to form an upper electrode 25. A part of the one surface of the glass substrate 21 is coated with a metal such as ITO so as to contact with the upper electrode 25 and thereby a picture element electrode 26 is formed. In the case where the upper electrode 25 is made of electroconductive oxide film, the upper electrode 25 itself can be also used as the picture element electrode 26.
A liquid crystal display apparatus using a TFD element is driven by applying a voltage to liquid crystal molecules via picture signals inputted through the TFD element and via vertical scanning signals inputted from transparent wirings formed on the substrates opposed to each other.
The most famous TFD liquid crystal display element is of a metal-insulator-metal (hereinafter abbreviated as MIM) constitution type wherein Ta.sub.2 O.sub.5 formed by anodization is used as a TFD element. The MIM element is relatively good in symmetry of voltage-ampere characteristic on a positive-voltage and a negative-voltage side on applying a voltage, and excellent in uniformity. Accordingly, it is only the MIM element among TFD liquid crystal display elements that was entered into the market.
The symmetry of the voltage-ampere characteristic, however, is not complete even in the MIM element yet, and therefore flickering and image fixing, which are causes of after-image etc., occur easily. A solution proposed in order to improve the symmetry of voltage-ampere characteristic is to connect two MIM elements with each other in series in a backward direction, that is a back-to-back constitution. However, in prior art (Japanese Examined Patent Publication JP(B2) 3-5072 (1991) etc.), it is impossible to eliminate dry etching for removing an anodized Ta.sub.2 O.sub.5 film. That causes the lowering of throughput and as a result leads to the increase of manufacturing cost.
Further, also a method for manufacturing a diode with a back-to-back constitution wherein an anodized film is employed instead of dry etching has been invented (Japanese Unexamined Patent Publication JPA 5-27269 (1993)). However, in the method, since a wiring, a connection and a MIM element are formed at the same time, the connection is formed in the quite same film thickness as those of the wiring and the MIM element. Accordingly, when the wiring and the MIM element are separated, not only the anodization from a side direction, but also applying a high voltage is required. At that time, peeling of resist and the like may be caused. Additionally, the anodized film has a limitation in thickness and a metallic portion between the wiring and the MIM element to be oxidized should be a line with a width of 1 .mu.m or less in order to be subjected to oxidation. That is very difficult in view of the patterning accuracy and is not practical.